This invention relates to a process for making a blend of olefin copolymers having different molecular weights and, more particularly, to such a process in which gaseous hydrogen is recovered for reuse as a chain-transfer agent in a single monomer recovery and recycle system for making blends of high and low molecular weight olefin copolymers.
Copolymers prepared by copolymerizing ethylene with a higher olefin, e.g., propylene and, optionally, nonconjugated dienes, are well known commercial products. Efficient manufacture of these products requires a copolymer synthesis process which involves continuously feeding a coordination catalyst and monomers to a reactor having a liquid phase where copolymerization occurs, continuously removing a portion of this liquid (containing a mixture of copolymer, unreacted monomers, catalyst residue and solvent) in order to isolate the copolymer product, recovering unreacted monomers and solvent and recycling them to the reactor.
It is known that blends of high and low molecular weight olefin copolymers display better processing properties than do the high or low molecular weight copolymers themselves. Therefore, suitable copolymer components for these blends are prepared by using chain-transfer agents such as hydrogen in the reactor liquid phase during copolymerization therein to control the molecular weight of the copolymer being formed. The chain-transfer agent terminates the growth of the copolymer molecule by displacing the coordination catalyst without deactivating it; a catalyst site is thereby made available for making a new copolymer molecule. The higher the concentration of the chain-transfer agent in the reactor liquid phase, the lower the molecular weight of the resulting polymer. Chain-transfer agents are especially attractive to use to control molecular weight of copolymers because they do not alter the copolymer composition.
In order to produce blends of olefin copolymers having different molecular weights usually two continuous reactors, operating simultaneously, are used in which the hydrogen concentrations in the liquid phases are different. Usually, at least about 10% by weight of the total copolymer blend is made in each reactor.
After the polymer blend is made it can be separated from unreacted monomers and hydrogen in a conventional manner by flashing, for example, in a stripper. However, a serious problem arises concerning reuse of hydrogen and unreacted monomers. Due to the large proportion of monomers in the resulting stripper off-gas mixture, one cannot split and recycle the gaseous mixture among the reactors in order to maintain the desired hydrogen concentrations in the liquid reaction zones and, at the same time, maintain the monomer concentration ratios desired therein. If the off-gas stream is split to apportion the hydrogen properly, incorrect monomer ratios result; if the stream is split to apportion the monomers properly, then hydrogen distribution is unsatisfactory. There is no practical or commercially feasible technique for apportioning the hydrogen in the off-gas stream to a reactor in which high molecular weight polymer is being made without first purging to the atmosphere most of the stripper off-gas stream being recycled, thereby losing not only hydrogen but substantial amounts of increasingly scarce and expensive unreacted monomers. Accordingly, there is a need for a process in which hydrogen can be recovered and recycled in preselected concentrations to separate reactors to regulate molecular weight of the copolymer components of a blend and at the same time permit independent product composition control so that a preselected ratio of monomer units is contained in the copolymers.